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<tr class="t-nv"><td colspan="5"> <strong class="selflink">内存模型与数据竞争</strong></td></tr>
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<p>为 C 抽象机的目的，定义计算机内存存储的语义。
</p><p>可用于 C 程序的数据存储（内存）是一个或多个连续<i>字节</i>的序列。内存中每个字节拥有唯一的<i>地址</i>。
</p>
<h3><span class="mw-headline" id=".E5.AD.97.E8.8A.82">字节</span></h3>
<p><i>字节</i>是内存的最小可寻址单元。它定义为一系列连续的位，足以保有任何<i>基础执行字符集</i>（要求 <a href="translation_phases.html" title="c/language/translation phases">96 个字符</a>是单字节）。 C 支持大小为 8 位或更多的字节。
</p><p><span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">char</span></span></span> 、 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 及 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> <a href="type.html" title="c/language/types">类型</a>的存储和<a href="object.html" title="c/language/object">值表示</a>都使用一个字节。字节的位数可以用 <span class="t-lc"><a href="../types/limits.html" title="c/types/limits">CHAR_BIT</a></span> 访问。
</p><p>对于用字节表示其他基础类型的（包含大端与小端内存布局），见<a href="object.html#.E5.AF.B9.E8.B1.A1.E8.A1.A8.E7.A4.BA" title="c/language/object">对象表示</a>。
</p>
<h3><span class="mw-headline" id=".E5.86.85.E5.AD.98.E4.BD.8D.E7.BD.AE">内存位置</span></h3>
<p><i>内存位置</i>是 
</p>
<ul><li> 一个<a href="type.html#.E5.AF.B9.E8.B1.A1.E7.BB.84.E5.88.AB" title="c/language/types">标量类型</a>（算术类型、指针类型、枚举类型）的对象
</li><li> 或非零长<a href="bit_field.html" title="c/language/bit field">位域</a>的最大连续序列
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw4">char</span> a<span class="sy4">;</span>     <span class="co1">// 内存位置 #1</span>
    <span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">5</span><span class="sy4">;</span>  <span class="co1">// 内存位置 #2</span>
    <span class="kw4">int</span> c <span class="sy4">:</span> <span class="nu0">11</span>, <span class="co1">// 内存位置 #2 （连续）</span>
          <span class="sy4">:</span> <span class="nu0">0</span>,
        d <span class="sy4">:</span> <span class="nu0">8</span><span class="sy4">;</span>  <span class="co1">// 内存位置 #3</span>
    <span class="kw1">struct</span> <span class="br0">{</span>
        <span class="kw4">int</span> ee <span class="sy4">:</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="co1">// 内存位置 #4</span>
    <span class="br0">}</span> e<span class="sy4">;</span>
<span class="br0">}</span> obj<span class="sy4">;</span> <span class="co1">// 对象“ obj ”由 4 个分离的内存位置组成</span></pre></div></div>
 <table class="t-rev-begin">
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<h3> <span class="mw-headline" id=".E7.BA.BF.E7.A8.8B.E5.8F.8A.E6.95.B0.E6.8D.AE.E7.AB.9E.E4.BA.89">线程及数据竞争</span></h3>
<p>执行的线程是一个程序中的控制流，它以调用顶层函数 <span class="t-lc"><a href="../thread/thrd_create.html" title="c/thread/thrd create">thrd_create</a></span> 或其他方法起始。
</p><p>任意线程可潜在地访问程序中的任意对象（拥有自动及线程局域<a href="storage_duration.html" title="c/language/storage duration">存储期</a>的对象仍能通过指针被另一线程访问）。
</p><p>执行的不同线程始终允许同时访问（读或修改）不同的<i>内存位置</i>，这没有冲突和同步要求（注意同时更新二个同一结构体内的非原子位域是不安全的，若所有声明于其间的成员亦为（非零长）位域，不管那些插入的位域大小是多少）。
</p><p>一个表达式的<a href="eval_order.html" title="c/language/eval order">求值</a>写入一个内存位置，而另一求值读取或修改同一内存位置时，我们称这两个表达式<i>冲突</i>。拥有两个冲突表达式的程序有<i>数据竞争</i>，除非
</p>
<ul><li> 两个冲突求值是<a href="atomic.html" title="c/language/atomic">原子操作</a>
</li><li> 一个冲突求值<i>先发生于</i>另一个（见 <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order</a></span> ）
</li></ul>
<p>若发生数据竞争，则程序行为未定义。
</p><p>（特别是， <span class="t-lc"><a href="../thread/mtx_unlock.html" title="c/thread/mtx unlock">mtx_unlock</a></span> 与另一线程的 <span class="t-lc"><a href="../thread/mtx_lock.html" title="c/thread/mtx lock">mtx_lock</a></span> <i>同步</i>，从而<i>先发生于</i>后者，这使得可以用互斥锁保证排除数据竞争）
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">本节未完成<br>原因：一或二个小示例 </td></tr></table>
<h3> <span class="mw-headline" id=".E5.86.85.E5.AD.98.E9.A1.BA.E5.BA.8F">内存顺序</span></h3>
<p>线程从一个内存位置读取值时，它可能看到初始值、被同一线程写入的值，或被其他线程写入的值。关于线程所做的写入对其他线程变得可见的顺序细节，见 <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order</a></span> 。
</p>
</td>
<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 3.6 byte (p: 4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.14 memory location (p: 5)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 5.1.2.4 Multi-threaded executions and data races (p: 17-21)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 3.6 byte (p: 4)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 1.6 DEFINITIONS OF TERMS 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table></div>
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